1In 10-inch nonstick skillet, melt 2 tablespoons butter over medium-high heat. Stir in onion and garlic to coat with butter. Cook uncovered about 5 minutes, stirring occasionally, until onion is softened. Reduce heat to medium-low. Cook uncovered 35 to 40 minutes, stirring occasionally, until onion is light golden brown. Do not let onion or garlic burn. (Onions will shrink during cooking.)
2Meanwhile, fill large pot with 5 quarts water; heat to boiling over high heat. When water is boiling, add 1 tablespoon salt if desired. When water returns to boil, add pasta. Stir immediately to make sure it doesn't stick to itself or pan. Boil pasta 9 to 10 minutes or until cooked through but still slightly firm. Drain in colander. There is no need to shake off excess moisture.
3Add onion mixture to pot. Increase heat to medium and add 1 cup of the whipping cream and 1/4 cup butter. Cook, stirring occasionally, until butter melts and cream simmers. Stir in cheese, salt, pepper and nutmeg. Add drained pasta and remaining 3/4 cup whipping cream. Cook and stir until pasta is coated and sauce is hot.